Pre-compensation of the phase shifting error

ABSTRACT

In this disclosure, methods for pre-compensation of the phase shifting error, and apparatuses for the same are disclosed. In one example, a device performs precoding of a digital signal, while acquiring information on an error caused by a phase shifting of the precoding. Then, the device performs phase compensation on the digital signal based on the acquired information. Here, the phase compensation may compensate different amount of phase based on an amount of the phase shifting of the precoding. This phase compensated-digital signal is converted to an analog signal, and is transmitted to a receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2014/003938, filed on May 2, 2014,which claims the benefit of U.S. Provisional Application No. 61/901,454,filed on Nov. 8, 2013, the contents of which are all hereby incorporatedby reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to methods for pre-compensation of the phase shiftingerror, and apparatuses for the same.

BACKGROUND ART

As an example of a wireless communication system to which the presentinvention is applicable, a 3rd generation partnership project (3GPP)long term evolution (LTE) communication system will be schematicallydescribed.

FIG. 1 is a schematic diagram showing a network structure of an evolveduniversal mobile telecommunications system (E-UMTS) as an example of awireless communication system. The E-UMTS is an evolved form of thelegacy UMTS and has been standardized in the 3GPP. In general, theE-UMTS is also called an LTE system. For details of the technicalspecification of the UMTS and the E-UMTS, refer to Release 7 and Release8 of “3rd Generation Partnership Project; Technical Specification GroupRadio Access Network”.

Referring to FIG. 1, the E-UMTS includes a user equipment (UE), anevolved node B (eNode B or eNB), and an access gateway (AG) which islocated at an end of an evolved UMTS terrestrial radio access network(E-UTRAN) and connected to an external network. The eNB maysimultaneously transmit multiple data streams for a broadcast service, amulticast service and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in oneof bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides adownlink (DL) or uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be set to provide differentbandwidths. The eNB controls data transmission or reception to and froma plurality of UEs. The eNB transmits DL scheduling information of DLdata to a corresponding UE so as to inform the UE of a time/frequencydomain in which the DL data is supposed to be transmitted, coding, adata size, and hybrid automatic repeat and request (HARQ)-relatedinformation. In addition, the eNB transmits UL scheduling information ofUL data to a corresponding UE so as to inform the UE of a time/frequencydomain which may be used by the UE, coding, a data size, andHARQ-related information. An interface for transmitting user traffic orcontrol traffic may be used between eNBs. A core network (CN) mayinclude the AG and a network node or the like for user registration ofUEs. The AG manages the mobility of a UE on a tracking area (TA) basis.One TA includes a plurality of cells.

Although wireless communication technology has been developed to LTEbased on wideband code division multiple access (WCDMA), the demands andexpectations of users and service providers are on the rise. Inaddition, considering other radio access technologies under development,new technological evolution is required to secure high competitivenessin the future. Reducing interference, decrease in cost per bit, increasein service availability, flexible use of frequency bands, a simplifiedstructure, an open interface, appropriate power consumption of UEs, andthe like are required.

DISCLOSURE Technical Problem

Accordingly, the present invention is directed to methods forpre-compensation of the phase shifting error, and apparatuses for thesame that substantially obviates one or more problems due to limitationsand disadvantages of the related art.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

Technical Solution

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,methods and apparatuses are provided.

In one aspect, a method for a transmitting device transmitting, signalsin a mobile communication system using multiple antennas, the methodcomprising: precoding a digital signal; acquiring information on anerror caused by a phase shifting of the precoding; performing a phasecompensation on the digital signal based on the acquired information,wherein the phase compensation compensates different amount of phasebased on an amount of the phase shifting of the precoding; convertingthe phase compensated-digital signal to an analogue signal; andtransmitting the analogue signal to a receiver, is provided.

The transmitting device may comprise ‘O’ antenna units, and each of the‘O’ antenna units may comprise ‘N’ antennas, where the ‘O’ and the ‘N”are natural numbers greater than 1.

The amount of the phase shifting of the precoding may be different foreach of ‘O’ antenna units.

The amount of the phase shifting of the precoding may be the same forthe ‘N’ antennas of one antenna unit.

The phase compensation may compensate the same amount of phase to the‘N’ antennas of one antenna unit and different amount of phase to eachof the ‘O’ antenna units based on the amount of the phase shiftingapplied to each of the ‘O’ antenna units.

The information on the error caused by the phase shifting may compriseamount of error per subcarrier set.

The phase compensation may compensate different amount of phase for eachsubcarrier set.

The information may be acquired based on feedback information from thereceiver.

In another aspect of the present invention, a device operating in awireless communication system, the device comprising: multiple antennaunits each of which comprises multiple antennas; a transceiver fortransmitting and receiving signals to and from another device using themultiple antennas of one or more of the multiple antenna units; and aprocessor connected to the transceiver and configured to performprecoding a digital signal to be transmitted, wherein the processoracquires information on an error caused by a phase shifting of theprecoding, performs a phase compensation on the digital signal based onthe acquired information, converts the phase compensated-digital signalto an analogue signal, and controls the transceiver to transmit theanalogue signal to the another device, wherein the phase compensationcompensates different amount of phase based on an amount of the phaseshifting of the precoding, is provided.

The number of the multiple antenna units may be ‘O’ and the number ofmultiple antennas of one antenna unit may be ‘N’, where the ‘O’ and the‘N” are natural numbers greater than 1.

The amount of the phase shifting of the precoding may be different foreach of ‘O’ antenna units.

The amount of the phase shifting of the precoding may be the same forthe ‘N’ antennas of one antenna unit.

The processor may compensate the same amount of phase to the ‘N’antennas of one antenna unit and different amount of phase to each ofthe ‘O’ antenna units based on the amount of the phase shifting appliedto each of the ‘O’ antenna units.

The information on the error caused by the phase shifting may compriseamount of error per subcarrier set.

The processor may compensate different amount of phase for eachsubcarrier set as the phase compensation.

Advantageous Effects

According to embodiments of the present invention, the network and theuser equipment can efficiently transmit and receive signals in awireless communication system.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system.

FIG. 2 is a diagram conceptually showing a network structure of anevolved universal terrestrial radio access network (E-UTRAN).

FIG. 3 is a diagram showing physical channels used in a 3GPP system anda general signal transmission method using the same.

FIG. 4 is a diagram to describe an antenna tilting system.

FIG. 5 is a diagram for one example of comparing an existing antennasystem and an active antenna system to each other.

FIG. 6 is a diagram for one example of forming a UE-specific beam basedon an active antenna system.

FIG. 7 is a diagram of a 2-dimensional (2D) beam transmission scenariobased on an active antenna system.

FIGS. 8-10 show various antenna array types for the present invention.

FIG. 11 is a diagram for explaining phase shifting error in a widebandtransmission system.

FIGS. 12 and 13 show examples of antenna system for employing theembodiments of the present invention.

FIGS. 14 and 15 show transmitting and receiving blocks employingmultiple phase shifters in OFDM system.

FIGS. 16 and 17 show an example of preferred embodiment forpre-compensating phase shifting errors.

FIG. 18 shows an example of one preferred embodiment of the presentinvention applying different phase shifting per subcarrier set.

FIGS. 19 and 20 show examples for estimating phase shifting errors usingvarious pilot types.

FIG. 21 is a block diagram of a communication apparatus according to anembodiment of the present invention.

BEST MODE

The configuration, operation and other features of the present inventionwill be understood by the embodiments of the present invention describedwith reference to the accompanying drawings. The following embodimentsare examples of applying the technical features of the present inventionto a 3rd generation partnership project (3GPP) system.

Although the embodiments of the present invention arc described using along term evolution (LTE) system and a LTE-advanced (LTE-A) system inthe present specification, they are purely exemplary. Therefore, theembodiments of the present invention are applicable to any othercommunication system corresponding to the above definition.

FIG. 2 is a diagram conceptually showing a network structure of anevolved universal terrestrial radio access network (E-UTRAN). An E-UTRANsystem is an evolved form of a legacy UTRAN system. The E-UTRAN includescells (eNB) which are connected to each other via an X2 interface. Acell is connected to a user equipment (UE) via a radio interface and toan evolved packet core (EPC) via an S1 interface.

The EPC includes a mobility management entity (MME), a serving-gateway(S-GW), and a packet data network-gateway (PDN-GW). The MME hasinformation about connections and capabilities of UEs, mainly for use inmanaging the mobility of the UEs. The S-GW is a gateway having theE-UTRAN as an end point, and the PDN-GW is a gateway having a packetdata network (PDN) as an end point.

FIG. 3 is a diagram showing physical channels used in a 3GPP system anda general signal transmission method using the same.

When a UE is powered on or enters a new cell, the UE performs an initialcell search operation such as synchronization with an eNB (S401). Tothis end, the UE may receive a primary synchronization channel (P-SCH)and a secondary synchronization channel (S-SCH) from the eNB to performsynchronization with the eNB and acquire information such as a cell ID.Then, the UE may receive a physical broadcast channel from the eNB toacquire broadcast information in the cell. During the initial cellsearch operation, the UE may receive a downlink reference signal (DL RS)so as to confirm a downlink channel state.

After the initial cell search operation, the UE may receive a physicaldownlink control channel (PDCCH) and a physical downlink control channel(PDSCH) based on information included in the PDCCH to acquire moredetailed system information (S402).

When the UE initially accesses the eNB or has no radio resources forsignal transmission, the UE may perform a random access procedure (RACH)with respect to the eNB (steps S403 to S406). To this end, the UE maytransmit a specific sequence as a preamble through a physical randomaccess channel (PRACH) (S403) and receive a response message to thepreamble through the PDCCH and the PDSCH corresponding thereto (S404).In the case of contention-based RACH, the UE may further perform acontention resolution procedure.

After the above procedure, the UE may receive PDCCH/PDCCH from the eNB(S407) and may transmit a physical uplink shared channel(PUSCH)/physical uplink control channel (PUCCH) to the eNB (S408), whichis a general uplink/downlink signal transmission procedure.Particularly, the UE receives downlink control information (DCI) throughthe PDCCH. Here, the DCI includes control information such as resourceallocation information for the UE. Different DCI formats are definedaccording to different usages of DCI.

Control information transmitted from the UE to the eNB in uplink ortransmitted from the eNB to the UE in downlink includes adownlink/uplink acknowledge/negative acknowledge (ACK/NACK) signal, achannel quality indicator (CQI), a precoding matrix index (PMI), a rankindicator (RI), and the like. In the case of the 3GPP LTE system, the UEmay transmit the control information such as CQI/PMI/RI through thePUSCH and/or the PUCCH.

In one embodiment of the present invention to which the above-describedsystem is applicable, a compensation method considering characteristicsof a phase shifter upon analog beamforming at a transmitter including atwo-dimensional (2D) antenna array including a plurality of antennas isprovided. A phase shifter shifts the phase of an analog RF signalpassing through a digital-to-analog converter (DAC) and may shift thephase in a desired direction in a relatively narrow frequency band.However, in consecutive wideband transmission of several hundreds of MHzor more, the phase shifting values of both ends of the frequency bandmay be different from that of the center of the frequency band. Inaddition, phase shifting error may vary according to angle set per phaseshifter. Since it is impossible to divide a signal, which has alreadybeen converted into an RF analog signal, per band and to enable thedivided signal to pass through the phase shifter, in the presentembodiment, a process of pre-compensating for a phase shiftingdifference per band in a digital signal processing block before passingthrough a DAC of a transmitter is proposed.

In the detailed embodiment, a method of estimating a phase differencevalue generated per band when phase shifting is performed by a phaseshifter, at a transmitter for performing wideband analog beamforming,and pre-compensating for the phase difference in a digital signalprocessing procedure is proposed. In addition, in another aspect of thepresent invention, a method of estimating a phase difference of a phaseshifter and an antenna structure capable of pre-compensating for thephase difference are proposed.

For this purpose, first, active antenna system and 3D beamforming schemeare explained.

FIG. 4 is a diagram to describe an antenna tilting system.

Particularly, FIG. 4(a) shows an antenna structure to which an antennatilting is not applied. FIG. 4(b) shows an antenna structure to which amechanical tilting is applied. And, FIG. 4(c) shows an antenna structureto which both a mechanical tilting and an electrical tilting areapplied.

Comparing FIG. 4(a) and FIG. 4(b) to each other, regarding a mechanicaltilting, as shown in FIG. 4(b), it is disadvantageous in that a beamdirection is fixed in case of an initial installation. Moreover,regarding an electrical tilting, as shown in FIG. 4(c), despite that atilting angle is changeable using an internal phase shift module, it isdisadvantageous in that a very restrictive vertical beamforming isavailable only due to a substantially cell-fixed tilting.

FIG. 5 is a diagram for one example of comparing an existing antennasystem and an active antenna system to each other.

Particularly, FIG. 5(a) shows an existing antenna system, while FIG.5(b) shows an active antenna system.

Referring to FIG. 5, in an active antenna system, unlike an existingantenna system, each of a plurality of antenna modules includes activedevices such as a power amplifier, an RF module and the like. Hence, theactive antenna system is capable of controlling/adjusting a power andphase for each of the antenna modules.

In a generally considered MIMO antenna structure, a linear antenna(i.e., 1-dimensional array antenna) like a ULA (uniform linear array)antenna is taken into consideration. In this 1-dimensional arraystructure, a beam generable by beamforming exists in a 2-dimensionalplane. This applies to a PAS (passive antenna system) based MIMOstructure of an existing base station. Although vertical antennas andhorizontal antennas exist in the PAS based base station, since thevertical antennas are combined into one RF module, beamforming invertical direction is impossible but the above-mentioned mechanicaltilting is applicable only.

Yet, as an antenna structure of a base station evolves into AAS, anindependent RF module is implemented for each antenna in a verticaldirection, whereby a beamforming in a vertical direction is possible aswell as in a horizontal direction. Such a beamforming is called anelevation beamforming.

According to the elevation beamforming, generable beams can berepresented in a 3-dimensional space in vertical and horizontaldirections. Hence, such a beamforming can be named a 3-dimensional (3D)beamforming. In particular, the 3D beamforming is possible because the1D array antenna structure is evolved into a 2D array antenna structurein a plane shape. In this case, the 3D beamforming is possible in a 3Darray structure of a ring shape as well as in a planar-shaped antennaarray structure. The 3D beamforming is characterized in that an MIMOprocess is performed in a 3D space owing to antenna deployments ofvarious types instead of an existing 1D array antenna structure.

FIG. 6 is a diagram for one example of forming a UE-specific beam basedon an active antenna system.

Referring to FIG. 6, owing to the 3D beamforming, a beamforming ispossible in case that a user equipment moves back and forth as well asin case that the user equipment moves right and left to the basestation. Hence, it can be observed that a higher degree of freedom isprovided to a UE-specific beamforming.

Moreover, as a transmission environment using an active antenna based 2Darray antenna structure, an environment (O2I: outdoor to indoor) of atransmission from an outdoor base station to an indoor user equipment,an environment (indoor hotspot) of a transmission from an indoor basestation to an indoor user equipment or the like can be considered aswell as an environment of a transmission from an outdoor base station toan outdoor user equipment.

FIG. 7 is a diagram of a 2-dimensional (2D) beam transmission scenariobased on an active antenna system.

Referring to FIG. 7, assuming a real cell environment in which aplurality of various buildings exist within a cell, a base station needsto consider a vertical beam steering capability in consideration ofvarious user equipment heights in accordance with a building height aswell as a UE-specific horizontal beam steering capability. Consideringsuch a cell environment, it is necessary to reflect channelcharacteristics (e.g., radio shadow/path loss variation due to a heightdifference, fading characteristic change, etc.) considerably differentfrom an existing radio channel environment.

So to speak, a 3D beamforming, which is evolved from a horizontalbeamforming performed in a horizontal direction only based on an antennastructure of an existing linear ID array, indicates an MIMO processingscheme performed in a manner of being extended to and combined with anelevation beamforming or a vertical beamforming based on an antennastructure of multi-dimensional arrays including a planar array and thelike.

In addition to or instead of the above mentioned Adaptive antenna systemand 3D beam forming scheme, various antenna array types can be used.

FIGS. 8-10 show various antenna array types for the present invention.Specifically, FIG. 8 shows a concept of passive antenna array (CommonTRX+Common PA+Multiple N antenna). FIG. 9 shows a concept of activeantenna array (Common TRX+Multiple PA+Multiple N antenna). FIG. 10 showsa concept of multiple Active antenna array (Multiple TRX+MultiplePA+Multiple N antenna).

Unlike a general antenna transmitter/receiver configuration, in anactive array antenna, a power amplifier (PA) and a phase shifter may becoupled to each antenna transmitter. Accordingly, three types of activearray antennas shown in FIGS. 8 to 10 may be considered.

A common TRX means a transmission and reception signal processing block.That is, in a passive array of FIG. 8, one RF transmission signal isbranched into a plurality of antennas. Each antenna includes only aphase shifter without a PA. Accordingly, since phase shifting of thesame RF signal is performed equally or differently according to antennaelement, the same or individual phase shifting is possible.

An active antenna array has a structure in which phase shifters and PAsare coupled to antenna elements in 1:1 correspondence as shown in FIG.9. Since a common transmission and reception signal processing block isused, the same RF signal is branched similarly to FIG. 8. The activeantenna array is different from the passive array in that PAscorresponding in number to the number N of antenna elements areincluded.

A multiple active antenna array has a structure in which TRXs, phaseshifters and PAs corresponding in number of to the number N of antennaelements are coupled. Accordingly, the multiple active antenna array hasa more complex structure than those of the above-described antennaarrays but has highest flexibility for analog beam control.

The phase shifter used for the above-described antenna array systemswill now be described.

The phase shift (phase shifter) refers to a device for shifting a signalphase using an electrical or mechanical method. The phase shifter usedherein may be a fundamental module for driving the above-describedmassive antenna for beam control and phase shifting of the phase arrayantenna as shown in FIGS. 8 to 10. The method of shifting the phase atthe phase shifter will now be described.

The phase shift is added to a final RE signal processor after a basebandsignal subjected to digital signal processing is converted into ananalog signal by a digital-to-analog (DAC) converter and the analogsignal is processed. That is, in the phase shift, signal processing perfrequency band is impossible. The phase shifting method of the phaseshifter may include the following five methods.

1. Method of mechanically changing, the length of a line.

In a structure in which two metal coaxial lines overlap, one coaxialpipe expands and contracts while being inserted into and taken out ofthe other coaxial line.

Merit: The phase may be consecutively changed. Low loss.

Demerit: It takes significant time to change the phase due to amechanical method. Large size (including passive type and automaticmotor type)

2. Phase shift method 1 of electrically changing the length: Linechanging method

A plurality of transmission lines having different lengths is providedand the paths thereof are changed by a switch.

Merit: Small size. A phase shift time is very short.

Demerit: The phase value may not be consecutively changed (digital).Greater loss than in mechanical method.

E.g.) 4-bit phase shifter of a line changing method: The phase may bechanged from 0 to 337.5 in units of 22.5.

3. Phase shift method 2 of electrically changing the length: Reflectionmethod

Similarly to the principle that tight is reflected from some place suchthat the phase thereof is changed, an electrical signal is reflectedfrom an impedance change point such that the phase thereof is changed.

Merit: The insertion phase may be adjusted according to a value of anelement connected to a middle part of a transmission line.

Demerit: Insertion loss deteriorates and impedance characteristics alsodeteriorate.

4. Phase shift method 3 of electrically changing the length: Loaded linetype, hybrid coupled type

This is frequently used for a digital type phase shifter.

Loaded line type: This is used for a phase shifter having a phase shiftamount of 45° or less.

Hybrid Coupled Type: This is used for a phase shifter having a phaseshift amount of 45° or more.

The phase is changed using reactance change when a PIN diode is turnedon/off.

5. Vector Modulator Phase Shifter

Method of obtaining a signal having a necessary phase by adjusting thelevels of two orthogonal components according to desired phase andmixing the two orthogonal components at a mixer

E.g.,) A=r∠0° (=r cos 0°+j r sin 0°)→[3 dB Amp] √2 r∠0°→[3 dB 90°Hybrid] r∠0°, r∠90°

→[Variable Attenuator] r cos Θ∠0°, r sin Θ∠90°→[Combiner] r∠Θ°

If the above-described antenna system is used, the below-described phaseshifting errors may occur. In one aspect of the present invention, amethod of compensating for phase shift errors which may occur in a phaseshifter for analog beamforming when performing wideband transmissionwill be described. Phase shifting of the phase shifter may be generallyperformed in a narrow band without any problems. That is, phase shiftingmay be performed with respect to an analog signal of a band of 5 MHz or10 MHz without any problems.

However, currently, basic bandwidth of high-frequency band transmissionfor designing a system based on a wideband frequency is several hundredsof MHz or several GHz. Thus, current design of the phase shifter is onlyvalid within several MHz or 10 MHz or less from the center frequencyf_(c). In this case, phase shift of an analog beam occurs according tothe phase value set in the phase shifter with respect to the centerfrequency but the value thereof may be changed with respect to thefrequency band other than the center frequency band.

FIG. 11 is a diagram for explaining phase shifting error in a widebandtransmission system.

As shown in FIG. 11, the phase is shifted by δ_(p) in a valid range fromthe center of the whole frequency bandwidth but the phase value isgradually changed in the range other than the valid range. This mayoccur upon wideband transmission. Since phase shifting is performedthrough the phase shift after the baseband signal is converted into ananalog signal. In this case, the phase of an analog beam is shiftedaccording to phase value set in the phase shifter with respect to thecenter frequency band but the phase value may be changed with respect tothe band other than the center frequency band.

FIGS. 12 and 13 show examples of antenna system for employing theembodiments of the present invention.

Fundamentally, if the phase shifter is used for analog beamforming,after digital signal processing has been finished, the transmittedsignal converted into the analog RF signal may be sent to M phaseshifters and antenna elements as shown in FIG. 12. In thebelow-described embodiment, assume that the antenna systems shown inFIGS. 12 and 13 are used.

In one embodiment of the present invention, assume that an analog RFsignal generated by a single TRX block is sent to M phase shifters and Mantenna elements set to have the same phase in each unit. In such astructure is assumed, the phase shifter should simultaneously set phaseshifting setting values δ_(p) with respect to the M elements. Morespecifically, since phase shifting error caused by the phase shiftingsetting value δ_(p) is not linear, the pre-compensation phase shiftingvalue should be set in consideration of both δ_(p) and a subcarrierindex “k”. For example, if the phase shifting setting value of the phaseshifter is p=30° or 45°, phase errors δ_(k) ^(p) caused in transmissionbands may be different (δ_(k) ³⁰≠δ_(k) ⁴⁵). Accordingly, in the presentinvention, pre-compensation for phase error, which should be compensatedfor per subcarrier according to the phase shifting setting value δ_(p),at a digital processing block based on different values δ_(k) ^(p) isproposed.

Accordingly, in the basic transmitter structure, as shown in FIG. 12, RFsignals, the phases of which are simultaneously shifted to the samephase by the phase shifters set to a single angle, may be generated or aseparate digital transmission/reception signal processing block may beprovided per unit including M elements to generate a mixture of RFsignals, the phases of which are shifted to various phases.

The embodiments of FIGS. 12 and 13 may be summarized as follows.

Approach 1 (FIG. 12): Single TRX+Multiple Phase-shifter with sameset-value.

Approach 2 (FIG. 13): Multiple TRX+‘M’ Phase-shifter with same set-value(δ_(p)=[δ_(p) ₁ ,δ_(p) ₂ ,δ_(p) ₃ , . . . , δ_(p) _(O) ])

-   -   Unit 1: Single TRX+‘M’ Phase-shifters with same δ_(p) ₁    -   Unit 2: Single TRX+‘M’ Phase-shifters with same δ_(p) ₂    -   Unit O: Single TRX+‘M’ Phase-shifters with same δ_(p) _(O)

Approach 2 may be regarded as an extension of Approach 1. Here, TRX₁ toTRX_(O) refer to blocks for generating the same transport blocks orinformation. In addition, each unit is virtualized via M antennas forperforming the same phase shifting. In this case, the phase of each unitis shifted by the same phase shifting value δ_(p) using the M phaseshifters and then each unit is virtualized, thereby having thetransmission/reception structure shown in FIGS. 14 and 15.

FIGS. 14 and 15 show transmitting and receiving blocks employingmultiple phase shifters in OFDM system.

As shown in FIG. 14, a phase shifter is located in an OFDM transmissionblock and phase shifting is performed via a phase shifter after atransmission symbol subjected to OFDM modulation is converted into ananalog RF signal. Assume that, in the below-described equation, awireless channel is independent between each antenna element and areceive antenna.

A discrete OFDM signal subjected to OFDM modulation may be expressed byEquation 1 below.

$\begin{matrix}{{{x_{l}\lbrack n\rbrack} = {\sum\limits_{k = 0}^{N - 1}\;{{X_{l}\lbrack k\rbrack}e^{j\; 2\pi\;{kn}\text{/}N}}}}{{n = 0},1,2,3,\ldots\mspace{14mu},{N - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

where, x_(l)[n] denotes an n^(th) sample (time domain) obtained bysampling an l^(th) OFDM symbol and X_(l)[k] denotes a data symbol(frequency domain) transmitted on a k^(th) subcarrier of an l^(th) OFDMsymbol. In addition, l denotes an OFDM symbol index, k denotes asubcarrier index, n denotes a sample index of a discrete time domain,and N denotes FFT size, that is, the total number of subcarriers.

At this time, when the analog OFDM symbol passing through the phaseshifter is expressed in discrete signal form, Equation 2 below isobtained.

$\begin{matrix}{{{x_{l}^{\prime}\lbrack n\rbrack} = {{\sum\limits_{k = 0}^{N - 1}\;{{X_{l}\lbrack k\rbrack}e^{j\; 2\pi\;{kn}\text{/}N}e^{j\; 2{\pi{({\delta_{p} + \delta_{k}^{p}})}}}}} = {\sum\limits_{k = 0}^{N - 1}\;{{X_{l}\lbrack k\rbrack}e^{j\; 2{\pi{({{{kn}\text{/}N} + \delta_{p} + \delta_{k}^{p}})}}}}}}}\mspace{79mu}{{n = 0},1,2,3,\ldots\mspace{14mu},{N - 1}}\mspace{79mu}{{p = 0},1,2,3,{{\ldots\mspace{20mu} O} - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

where, x_(l)[n] denotes an n^(th) sample (time domain) obtained bysampling an l^(th) OFDM symbol passing through the phase shifter,y_(l)[n] denotes an n^(th) sample (time domain) of an l^(th) receivedOFDM symbol, and Y_(l)[k] denotes a data symbol (frequency domain) on ak^(th) subcarrier of an l^(th) received OFDM symbol. In addition, pdenotes a TRX unit index (an index of a TRX, to which the phase shifteris connected, when performing phase shifting using the phase shifter), Odenotes a total number of antenna ports (=TRX units), N denotes a FFTsize, that is, a total number of subcarriers, δ_(p) denotes a phaseshifting value of the phase shifter of TRX “p” and δ_(k) denotes phaseshifting error of the k^(th) subcarrier of the phase shifter of TRX “p”.

That is, in Equation 2, two variables are added: one is a phase shiftingvalue δ_(p) set in the phase shifter and the other is phase shiftingerror δ_(k) ^(p) caused by the phase shifter. Shifting error δ_(k) ^(p)varies according to phase shifting setting value δ_(p) and subcarrier, asubcarrier index ‘k’ is included in δ_(k) ^(p).

The received signal before last OFDM demodulation of FIG. 15 may beexpressed by a discrete signal shown in Equation 3 and is expressed byan OFDM demodulation symbol as shown in Equation 4 when the signal doesnot pass through the phase shifter.

$\begin{matrix}{{{y_{l}\lbrack n\rbrack} = {{{{x_{l}\lbrack n\rbrack}*{h_{l}\lbrack n\rbrack}} + {z_{l}\lbrack n\rbrack}} = {{\sum\limits_{m = 0}^{\infty}\;{{h_{l}\lbrack m\rbrack}{x_{l}\left\lbrack {n - m} \right\rbrack}}} + {z_{l}\lbrack n\rbrack}}}}\mspace{79mu}{{n = 0},1,2,3,\ldots\mspace{14mu},{N - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack \\{\mspace{79mu}{{Y_{l}\lbrack k\rbrack} = {{{H_{l}\lbrack k\rbrack}{X_{l}\lbrack k\rbrack}} + {Z_{l}\lbrack k\rbrack}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

where, h_(l)[n], z_(l)[n] denotes a wireless channel impulse functionand AWGN noise and H_(l)[k], Z_(l)[k] denotes a frequency response andnoise of a channel in a k^(th) subcarrier.

However, the received signal is demodulated in the form of Equation 5 ifthe signal passes through the phase shifter.Y _(l) [k]=H _(l) [k]X _(l) [k]e ^(j2π(δ) ^(p) ^(+δ) ^(k) ^(p) ⁾ +Z _(l)[k]  [Equation 5]

That is, the phase shifting error δ_(k) and the phase shifting settingvalue δ_(p) of an angle of a received signal coexist per subcarrier.

In one embodiment for solving such a problem, pre-compensation precodingis performed according to the phase shifting setting value in order tocompensate for phase errors occurring upon phase shifting of the phaseshifter.

FIGS. 16 and 17 show an example of preferred embodiment forpre-compensating phase shifting errors.

In the present embodiment, as shown in FIG. 16, phase shifting error iscompensated for before a signal is branched into M antenna elements(including the phase shifter) by adding a pre-compensation block beforethe signal is branched into antennas by a digital processing block topass through an IFFT block. Accordingly, phase shifter phasecompensation in each subcarrier may be expressed by Equation 6. At thistime, after pre-compensating precoding, the digital signal is mapped toa single antenna port. Thereafter, the set phase shifting value is inputto M phase shifters connected to M antenna elements as shown in FIG. 17.

Accordingly, the antenna port mapping number of FIG. 16 is equal to thetotal number “O” of TRX units. The branched signals are virtualized viathe “M” antenna elements and analog beamforming is performed through thephase shifter. Accordingly, the phase shifting value δ_(p) of analogbeamforming is set per antenna port.

For example, in FIG. 17, assume that the phase shifting setting value isset to δ_(p=0)=30° per antenna in antenna port=0, is set to δ_(p=1)=45°per antenna in antenna port=1, and is set to δ_(p=O)=90° per antenna inantenna port=“O”. This setting value refers to a value commonly set withrespect to the phase shifters connected to the M antenna elements.Accordingly, in pre-compensating precoding, phase shifting δ_(k) ^(p)for the phase shifting value δ_(p) per antenna port is commonlycompensated for.

In the present embodiment, a signal, to which phase shifter phasecompensation is applied in each subcarrier, may be expressed by Equation6.y=HF _(PS) Fx+z  [Equation 6]

where, y denotes a received signal vector N_(r)×1 in a k^(th)subcarrier, H denotes an N_(r)×N_(r) channel matrix in a k^(th)subcarrier, F_(PS) denotes an N_(r)×N_(r) phase shifter errorcompensation matrix (diagonal matrix) in a k^(th) subcarrier, F denotesa general N_(r)×ν precoding matrix for beamforming in a k^(th)subcarrier, x denotes a transmitted signal vector ν×1 in a k^(th)subcarrier, z denotes an AWGN noise vector N_(r)×1 in a k^(th)subcarrier, p denotes an antenna port index (=TRX unit index), that is,the number of TRX units (p=0, 1, 2, 3, . . . O−1).

At this time, each vector and matrix may be expressed by Equation 7.

$\begin{matrix}{\begin{bmatrix}y^{(0)} \\\vdots \\y^{({{Nr} - 1})}\end{bmatrix} = {\begin{bmatrix}h_{11} & h_{12} & \cdots & h_{1\;{Nt}} \\h_{21} & h_{22} & \cdots & h_{2\;{Nt}} \\\vdots & \vdots & \ddots & \vdots \\h_{{Nr}\; 1} & h_{{Nr}\; 2} & \cdots & h_{NrNt}\end{bmatrix}{\quad{\begin{bmatrix}e^{{- j}\; 2{\pi\delta}_{k}^{p}} & 0 & \cdots & 0 \\0 & e^{{- j}\; 2{\pi\delta}_{k}^{p}} & \cdots & 0 \\\vdots & \vdots & \ddots & \vdots \\0 & 0 & \cdots & e^{{- j}\; 2{\pi\delta}_{k}^{p}}\end{bmatrix}{\quad{{\begin{bmatrix}F_{11} & F_{12} & \cdots & F_{{Nt},v} \\F_{21} & F_{22} & \cdots & F_{{Nt},v} \\\vdots & \vdots & \ddots & \vdots \\F_{{Nt}\; 1} & F_{{Nt}\; 2} & \cdots & F_{{Nt},v}\end{bmatrix}\begin{bmatrix}x^{(0)} \\\vdots \\x^{({v - 1})}\end{bmatrix}} + \begin{bmatrix}z^{(0)} \\\vdots \\z^{({{Nr} - 1})}\end{bmatrix}}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

That is, since phase error is corrected by the phase shifting settingvalue δ_(p) set per antenna port, a phase error compensation matrix ofthe phase shifter has a diagonal matrix and a value thereof variesaccording to subcarrier. In Equation 7, it should be noted that each rowdoes not correspond to an antenna of each unit but corresponds to eachunit.

The analog signal branched into the antennas via such a method issubjected to analog beamforming with a phase shifting value of δ_(p) setin each system after passing through a phase shifter. Through such amethod, the signal, the phase of which has already shifted beforepassing through the phase shifter, hasY_(l)[k]=H_(l)[k]X_(l)[k]e^(j2π(δ) ^(p) ^(+δ) ^(k) ^(p) ^(−δ) ^(k) ^(p)⁾+Z_(l)[k] and thus only a phase shifting value δ_(p) set by the phaseshifter remains.

Accordingly, a maximum number N_(r) of transmit antennas shown inEquation 7 is not a total number of antennas of a massive array antennabut is a maximum number of logical antenna ports. Accordingly, arelationship of N_(r)=O is satisfied.

FIG. 18 shows an example of one preferred embodiment of the presentinvention applying different phase shifting per subcarrier set.

More specifically, in the present embodiment, phase shifting is appliedin subcarrier set units having similar phase error of thepre-compensation block. Although only the phase shifting value δ_(p)intended by the phase shifter should be changed according to subcarrier,when the signal passes through the phase shifter, constant phase errorδ_(k) ^(p), which vary according to frequency band or k^(th) subcarrier,may occur according to the phase shifting value δ_(p).

As described above, if transmission bandwidth is several MHz or less, itmay be assumed that the same phase error occurs in the whole frequencyband or subcarriers and thus a relationship shown in Equation 8 belowmay be satisfied.δ_(k) ^(p)→δ₀ ^(p)=δ₁ ^(p)=δ₂ ^(p) . . . =δ_(N−1) ^(p)  [Equation 8]

However, if the bandwidth is several hundreds of MHz or several GHz,these values may not be approximated to one value. Therefore, it may beassumed that the same phase error occurs in a predetermined band or aset including a predetermined number “S” of subcarriers. Accordingly, inthe present embodiment, it may be assumed that a relationship shown inEquation 9 below is satisfied.

$\begin{matrix}\left. \delta_{k}^{p}\rightarrow\left\{ \begin{matrix}{{\delta_{0}^{p} = {\delta_{1}^{p} = {{\delta_{2}^{p}\mspace{14mu}\ldots} = \delta_{S - 1}^{p}}}}\mspace{124mu}} \\{{\delta_{S}^{p} = {\delta_{S + 1}^{p} = {{\delta_{S + 2}^{p}\mspace{11mu}\ldots} = \delta_{{2S} - 1}^{p}}}}\mspace{70mu}} \\{{\delta_{2S}^{p} = {\delta_{{2S} + 1}^{p} = {{\delta_{{2S} + 2}^{p}\mspace{11mu}\ldots} = \delta_{{3S} - 1}^{p}}}}\mspace{45mu}} \\\vdots \\{\delta_{N - S}^{p} = {\delta_{N - S + 1}^{p} = {{\delta_{N - S + 2}^{p}\mspace{11mu}\ldots} = \delta_{N - 1}^{p}}}}\end{matrix} \right. \right. & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

According to the proposals of the present invention, as shown in FIG.18, pre-compensation precoding described in antenna port=0 isapplicable. That is, the same phase correction value −δ_(k) ^(p) isapplied per constant subcarrier set.

In one embodiment of the present invention, a base station estimatesshifting error according to phase setting value of a phase shifter viacomparison between transmitted signals and derives phase shifting errorper subcarrier to be compensated for according to phase shifting settingvalue.

For phase compensation of the phase shifter, phase error caused by thephase shifter should be estimated. Here, an entity for performing analogbeamforming using a massive array antenna directly performs channelestimation. That is, when the base station performs analog beamformingusing the massive antenna, a per-band phase difference between signalspassing through the phase shifter may be directly compared.

Meanwhile, in one embodiment of the present invention, a method of, at aUE, which has acquired the phase setting value of the phase shifter,estimating phase shifting error, and feeding the phase shifting errorback to a base station is proposed. This method uses a general pilotbased channel estimation method which may be set with respect to allUEs. That is, a conventional pilot channel estimation method may bereused. In this method, all UEs may feed phase error back per subbandwith respect to the whole frequency band. This is because phase shiftingerrors caused by the phase shifter differ between subcarriers orsubcarrier sets, according to the present embodiment.

First, the UE performs pure channel estimation in a state in which thephase shift does not operate. At this time, assume that a wirelesschannel is estimated per subcarrier using a least-squares method shownin Equation 10, where X_(l)[k] denotes a signal via which a pilot signalis transmitted.

$\begin{matrix}{{{\hat{H}}_{l}\lbrack k\rbrack} = \frac{Y_{l}\lbrack k\rbrack}{X_{l}\lbrack k\rbrack}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

Thereafter, channel estimation, to which the phase shifting settingvalue is applied, is performed. This may be expressed by Equation 11.

$\begin{matrix}{{\hat{H_{l,{ps}}}\lbrack k\rbrack} = {{{{\hat{H}}_{l}\lbrack k\rbrack}e^{j\; 2{\pi{({\delta_{p} + \delta_{k}^{p}})}}}} = \frac{Y_{l}\lbrack k\rbrack}{X_{l}\lbrack k\rbrack}}} & \left\lbrack {{Equation}\mspace{14mu} 11} \right\rbrack\end{matrix}$

At this time, if Equation 11 is divided by Equation 10, it is possibleto obtain a value including the phase shifting setting value δ_(p) givenper subcarrier “k” shown in Equation 12 below and the phase shiftingerror δ_(k) ^(p) occurring at that time.e ^(j2π(δ) ^(p) ^(+δ) ^(p) ^(p) ⁾ =Ĥ _(l,ps) [k]/Ĥ _(l) [k]  [Equation12]

At this time, it may be assumed that phase shifting error occurs in asubcarrier (k=N/2) corresponding to the center of the transmission band(δ_(k) ^(p)=0). In this case, pure phase shifting error is estimated viathe assumption shown in Equation 13.δ_(k) ^(p) →e ^(j2πδ) ^(k) ^(p) =Ĥ _(l,ps) [k]/Ĥ _(l) [k]/(Ĥ _(l,ps)[N/2]/Ĥ _(l) [N/2])  [Equation 13]

When the UE feeds such a result back to the base station, a soft valuemay be transmitted without change and the result may be fed back using aquantization method. For example, a section [0, 2π] may be quantizedinto N bits to have 2^(N) resolutions.

The base station uses the phase error value fed back by each UE or mayuse an average of the values fed back by all UEs.

Accordingly, the channel estimation order according to the presentembodiment may be set as follows.

Step 1: A pure wireless channel is estimated without phase shifting.(δ_(p)=0°)

Step 2: Pilot based channel estimation with the phase shifting settingvalue δ_(p=0) is performed. (δ_(p)+δ_(k) ^(p) simultaneous estimation)

Step 3: The phase estimation value of all subcarriers is subtracted fromthe estimation value of the subcarrier (N/2) corresponding to thetransmission band (see Equation 13)→The phase shifting error δ_(k) ^(p)is derived per subcarrier.

Step 4: The phase shifting error is fed back to the base station persubcarrier or per subcarrier set.

Meanwhile, in another embodiment of the present invention, a method ofsimultaneously setting pilot signals in the whole bandwidth andestimating phase error is proposed.

FIGS. 19 and 20 show examples for estimating phase shifting errors usingvarious pilot types.

Specifically, FIG. 19 is an example for estimating phase shifting errorsusing pilot pattern transmitted through the whole bandwidth, and FIG. 20is an example for estimating phase shifting errors using pilot patterntransmitted through a subband of the frequency band.

As described above, the phase shifter simultaneously changes the wholephase of the analog RF signal. Accordingly, the digital processing,block can compensate for the phase error of the whole bandwidth.Accordingly, each UE may derive phase error caused by the phase shifterbased on channel estimation for the whole system bandwidth, not based onbandwidth allocated thereto.

At this time, the base station may allocate a pilot signal to the wholetransmission bandwidth and perform estimation. For example, in the wholebandwidth, as shown in FIG. 19, a pilot pattern for the whole bandwidthmay be defined and the UE may perform interpolation between the detectedpilot signals and estimate phase shifting errors for all channels.

In contrast, as shown in FIG. 20, a method of setting a pilot signal persubband to perform estimation and periodically and repeatedlytransmitting the pilot signal may be used.

Unlike the above-described embodiments, each UE may derive phase errorcaused by the phase shifter based on channel estimation for the subbandallocated thereto.

At this time, the base station defines a pilot allocation pattern suchthat channel estimation for the whole bandwidth is possible or channelestimation for the whole bandwidth is possible over several symbols. Forexample, as shown in FIG. 20, different patterns are defined per subbandand channel and phase shifting error for the whole bandwidth may beestimated over three OFDM symbols. Even at this time, interpolationbetween pilot signals may be performed and phase shifting error for allchannels may be estimated.

As described above, the present invention proposes a compensation methodconsidering characteristics of a phase shifter upon analog beamformingat a transmitter including a 2-dimensional antenna array including aplurality of antennas is proposed.

Hereinafter, the configuration of an apparatus for implementing theabove-described methods will be described.

FIG. 21 is a block diagram of a communication apparatus according to anembodiment of the present invention.

The apparatus shown in FIG. 21 can be a user equipment (UE) and/or eNBadapted to perform the above mechanism, but it can be any apparatus forperforming the same operation.

As shown in FIG. 21, the apparatus may comprises a DSP/microprocessor(110) and RF module (transceiver; 135). The DSP/microprocessor (110) iselectrically connected with the transceiver (135) and controls it. Theapparatus may further include power management module (105), battery(155), display (115), keypad (120), SEM card (125), memory device (130),speaker (145) and input device (150), based on its implementation anddesigner's choice.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

INDUSTRIAL APPLICABILITY

While the above-described method has been described centering on anexample applied to the 3GPP LTE system, the present invention isapplicable to a variety of wireless communication systems in addition tothe 3GPP LTE system.

The invention claimed is:
 1. A method for a transmitting devicetransmitting signals via sets of subcarriers in a mobile communicationsystem using multiple antennas included in multiple antenna units, eachset of the sets of subcarriers corresponding to one of the multipleantenna units, the method comprising: precoding a digital signal;receiving information on phase errors with respect to a whole frequencyband, wherein the information on phase errors comprises a plurality ofphase shifting setting values, each phase shifting setting valuecorresponding to a specific subcarrier set of the sets of subcarrierscorresponding to the multiple antennas of the transmitting device;creating a phase error compensated—digital signal by performing a phaseerror compensation for each subcarrier set based on a corresponding oneof the plurality of phase shifting setting values, wherein an amount ofphase error compensated for each subcarrier set increases as a distancebetween a center frequency of the whole frequency band and a centerfrequency of the corresponding subcarrier set increases; converting thephase error compensated—digital signal to a phase shifted analoguesignal corresponding to each of the multiple antenna units; performinganalogue beamforming by phase shifting the analogue signal with respectto the whole frequency band using phase shifting setting valuescorresponding to each of the multiple antenna units, wherein amounts ofphase error compensated for a specific subcarrier set differ ascorresponding phase shifting setting values differ; and transmitting thephase shifted analogue signal to a receiver using each of the multipleantenna units.
 2. The method of claim 1, wherein the multiple antennaunits comprises ‘O’ antenna units and each of the ‘O’ antenna unitscomprises a plurality of antennas.
 3. The method of claim 2, wherein theamount of phase error compensated for each subcarrier set is differentfor each of the multiple antenna units.
 4. The method of claim 3,wherein the amount of phase error compensated for each subcarrier set isthe same for each antenna of an antenna unit of the multiple antennaunits.
 5. A device operating in a wireless communication system, thedevice configured to transmit signals via sets of subcarriers andcomprising: multiple antenna units each of which comprises multipleantennas, each set of the sets of subcarriers corresponding to one ofthe multiple antenna units; a transceiver for transmitting and receivingsignals to and from another device using the multiple antennas of themultiple antenna units; and a processor connected to the transceiver andconfigured to perform precoding a digital signal to be transmitted,wherein the processor receives information about phase errors withrespect to a whole frequency band, wherein the information on phaseerrors comprises a plurality of phase shifting setting values, eachphase shifting setting value corresponding to a specific subcarrier setof the sets of subcarriers corresponding to the multiple antennas of thetransmitting device, wherein the processor creates a phase errorcompensated—digital signal by performing a phase error compensation foreach subcarrier set based on a corresponding one of the plurality ofphase shifting setting values, wherein an amount of phase errorcompensated for each subcarrier set increases as a distance between acenter frequency of the whole frequency band and a center frequency ofthe corresponding subcarrier set increases, wherein the processorconverts the phase error compensated—digital signal to a phase shiftedanalogue signal corresponding to each of the multiple antenna units,wherein the processor performs analogue beamforming by phase shiftingthe analogue signal with respect to a whole frequency band using phaseshifting setting values corresponding to each of the multiple antennaunits, wherein amounts of phase error compensated for a specificsubcarrier set differ as corresponding phase shifting setting valuesdiffer, and wherein the processor controls the transceiver to transmitthe analogue signal to the another device using each of the multipleantenna units.
 6. The device of claim 5, wherein the amount of phaseerror compensated for each subcarrier set is different for each of themultiple antenna units.
 7. The device of claim 6, wherein the amount ofphase error compensated for each subcarrier set is the same for eachantenna of an antenna unit of the multiple antenna units.
 8. The methodof claim 1, wherein the amount of the phase error compensated for asubcarrier set including the center frequency of the whole frequencyband is
 0. 9. The device of claim 5, wherein the amount of the phaseerror compensated for a subcarrier set including the center frequency ofthe whole frequency band is 0.